On the role of spectral diffusion in single-molecule spectroscopy

This paper reports on measurements of fluorescence excitation spectra of single impurity molecules of terrylene in n-decane at T=1.7 K. Spectra measured within the same spectral interval but at consecutive instants of time exhibit zero-phonon (ZP) lines of single impurity molecules of several species, differing in the behavior of the line shape and frequency in time. On the one hand, one observes stable ZP lines, well approximated with a Lorentzian. On the other, one sees spectral features with a profile varying from one spectrum to another, with only individual fragments of such a profile allowing the Lorentzian approximation; such features are interpreted assuming the presence of unstable impurity molecules, the ZP lines of which display small (a few tens of MHz) spectral jumps with a time interval of about 10 s. Such molecules exhibit a substantial decrease in the spectral jump frequency within a measurement period of the order of 5000 s, which is attributed to a decrease in the contribution due to spectral diffusion resulting from sample structure relaxation.     

Hole burning spectroscopy with preliminary photoselection of impurity molecules

The photoselection of impurity molecules caused by spectral hole burning produces a specific spectral angular distribution of the molecules in low-temperature amorphous matrices: All the molecules, whose zero-phonon absorption lines lie within a selected narrow spectral interval, have a specified distribution of their orientations relative to the laboratory coordinate system. In this paper, the possibility of application of known methods of hole burning in this spectral interval is studied and the outlook for combination of these methods with preliminary photoselection is demonstrated by the example of the Stark effect.     

Recombination luminescence in irradiated silicon-effects of uniaxial stress and temperature variations†

Luminescence in irradiated silicon consists of a spectral group between 0.80 eV and 1.0 eV which seems to be independent of impurities while a lower energy group between 0.60 eV and 0.80 eV is seen only in pulled crystals. The small halfwidth and temperature dependence of the sharp zero-phonon lines observed in these spectra indicates that the luminescence arises from a bound-to-bound transition. A model is proposed for the transition mechanism. Stress data taken on the 0.79 eV zero-phonon line in pulled crystals can be fit by either a tetragonal [100] defect symmetry or by conduction band splitting effects. It is suggested that the 0.79 eV zero-phonon line and the 0.60 eV to 0.80 eV spectral group arise from the EPR G-15 center. Stress data on a zero-phonon line at 0.97 eV associated with the 0.80 eV to 1.0 eV spectral group can be explained by a trigonal [111] defect. The divacancy is tentatively suggested as responsible for this luminescence spectra.     

Coupling of nitrogen-vacancy centers to photonic crystal cavities in monocrystalline diamond

The zero-phonon transition rate of a nitrogen-vacancy center is enhanced by a factor of ～70 by coupling to a photonic crystal resonator fabricated in monocrystalline diamond using standard semiconductor fabrication techniques. Photon correlation measurements on the spectrally filtered zero-phonon line show antibunching, a signature that the collected photoluminescence is emitted primarily by a single nitrogen-vacancy center. The linewidth of the coupled nitrogen-vacancy center and the spectral diffusion are characterized using high-resolution photoluminescence and photoluminescence excitation spectroscopy.     

Low-temperature optical spectra and zero-phonon transition of color centers in γ-rayed lithium fluoride crystals

The zero-phonon lines on R_2, R~ , N_1 and R′_2 centers and some new narrow lines have been observed in bivalent-metal-doped and air-grown LiF crystals irradiated by γ-ray in the temperature range of 9.5—130K. The spectral properties and thermostabilities of the lines are investigated systematically at different temperatures.     

Phononenfreie Absorptions- und Emissionsübergänge in strahlungsverfärbten NaF-Kristallen

Inγ-irradiated NaF crystals at 5456, 5754, 5837, 6070 and 6146 å zero-phonon lines are observed in absorption and emission. These resonance lines are accompanied by multiphonon absorption and emission bands with their spectral positions nearly mirror symmetric with respect to the zero-phonon line position. By excitation spectra higher excited states are detected. Uniaxial stress splitting of the lines 5754 and 6070 å is studied in absorption and emission. The color centers associated with the lines are found to exhibit rhombic symmetry with rotation axes along 〈110〉 and 〈100〉 directions and monoclinic symmetry with a rotation axis along 〈110〉 direction. The transitions in both centers correspond to 〈110〉 dipoles. In the monoclinic center the transition between the ground and the second excited state seems to correspond to a 〈112〉 dipole.     

Persistent spectral hole-burning in electronic tranisitions of different types of lattice defects

Abstract

With a ring dye laser persistent spectral hole-burning was measured by fluorescence excitation spectroscopy in the zero-phonon lines of the ²ll_g ? ²ll_u transition in KI:S₂ ^? and of the ³H₄ → ³P₀ transition of a defect center with three D^? ions in SrF₂:Pr³⁺:D^?and also in the zero-phonon line at 462.5 nm of a color center in neutron-irradiated and annealed α-Al₂O₃. In all three lines holes with a width in the order of 1 GHz could be obtained at 1.5 K with single frequency laser. With broad-band laser in KI:S₂ ^? and SrF₂:Pr³⁺:D^? spectral redistribution within the full range of inhomogeneous broadening could be observed which are explained by a photophysical model based on barrier crossing in asymmetric double-well-potentials. In the case of sapphire a photobleaching process is dominant which is effective also for nonselective excitation, and the holes are thermoresistant up to 300 K.     

Low-temperature dynamics of amorphous polymers and evolution over time of spectra of single impurity molecules: I. Experiment

The optical dynamics of a doped amorphous system, tetra-tert-butylterrylene in amorphous polyisobutylene, has been experimentally studied by the spectra of single impurity molecules measured at temperatures of 2, 4.5, 7, and 15 K. The study of the temporal evolution of the fluorescence excitation spectra of the molecules under consideration made it possible to unambiguously establish the individual identity of the spectra of particular molecules and to analyze their multiplet structure. Repeated scanning of a selected spectral range with subsequent summation of the data made it possible to considerably reduce the errors that arise upon single scanning of the spectra of single molecules. The majority of the spectral trails detected were in agreement with the model of two-level systems. Jumps of spectral lines due to transitions in such systems were observed at all temperatures.     

Single-molecule spectroscopy of molecular aggregates at low temperature

We have conducted single-molecule spectroscopy of a fluorescent polyphenylene dendrimer consisting of four peripheral perylenemonoimides which serve as energy donors and a central terrylenediimide which is the energy acceptor. After selective excitation of the donors the low-temperature emission spectra of single dendrimers show the purely electronic zero-phonon line as the most prominent feature of the acceptor. These sharp emission lines are subjected to appreciable spectral shifts. Fluorescence excitation spectroscopy of individual dendrimers in the spectral region of the donor absorption allows to extract energy transfer rates for single donors within the dendrimer. Although the energy transfer from perylenemonoimide to terrylenediimide is quite efficient, energy transfer between two proximate perylenemonoimides might be a competing process. This is shown by experiments with molecular dimers, in which two perylenemonoimides are held at a short distance by a benzil spacer.     

Spin polarization induced by optical and microwave resonance radiation in a Si vacancy in SiC: A promising subject for the spectroscopy of single defects

Depending on the temperature, crystal polytype, and crystal position, two opposite schemes have been observed for the optical alignment of the populations of spin sublevels in the ground state of a Si vacancy in SiC upon irradiation with unpolarized light at frequencies of zero-phonon lines. A giant change by a factor of 2–3 has been found in the luminescence intensity of zero-phonon lines in zero magnetic field upon absorption of microwave radiation with energy equal to the fine-structure splitting of spin sublevels of the vacancy ground state, which opens up possibilities for magnetic resonance detection at a single vacancy.     

Site-selection optical spectra of bacteriochlorophyll and bacteriopheophytin in frozen solutions

Site-selection spectra of title compounds in different frozen solvents at 5K have been obtained: fluorescence spectra on selective excitation in vibronic as well as in the O-O absorption region and excitation spectra with narrow-band fluorescence recording. Frequencies of vibrational modes active in fluorescence and excitation spectra of bacteriochlorophyll a (BChl) and its metal-free derivative have been determined from these fine-line spectra. Most favourable vibronic line-to-background intensity ratios have been found in non-polar aprotic glassy environments (triethylamine, di-iso-amylether). The intensity of zero-phonon lines in microcrystallic protic matrices was low, indicating strong electron-phonon coupling. The vibrational frequencies of the excited electronic state characteristic of axially (at Mg atom) mono- and disolvated BChl species have been identified. Narrow spectral holes of about 20% of the initial absorption could be burned with ≈10^-4 quantum yield within the O-O band of BChl and bacteriopheophytin.     

Specific features in the vibronic fluorescence spectra of analogues of 1,4-bis(5-phenyl-2-oxazolyl)benzene with oxadiazole and furan rings

The fine-structure fluorescence and fluorescence excitation spectra of conjugated chain compounds, namely, 2,5-bis(5-phenyl-1,3,4-oxadiazol-2-yl)furan (PDFDP) and 2,5-bis[5-(2,4-dimethylphenyl)-1,3,4-oxadiazol-2-yl]furan (XDFDX), were obtained by the Shpolskii method in an n-octane matrix at a temperature of 4.2 K. These spectra were simulated by representing the band of each of the vibronic transitions as the sum of a zero-phonon line and a phonon wing with the corresponding parameters, such as the half-widths of the spectral lines and the Debye-Waller factors. The results obtained made it possible to estimate the relative intensities of the vibronic transitions between the S ₀ and S*₁ states. The anharmonicity revealed in the conjugate spectra of fluorescence and fluorescence excitation of the PDFDP and XDFDX compounds was explained in terms of the interference of the Franck-Condon and Herzberg-Teller interactions occurring in the molecules under investigation. The influence of the substitution of the furan heterocycle (F) for the central benzene ring (P) in 1,4-bis-(5-phenyl-2-oxadiazolyl)benzene (PDPDP) on the parameters of the intramolecular interactions responsible for the formation of the vibronic spectra was considered.     

Electron-phonon and vibronic couplings in the FMO bacteriochlorophyll a antenna complex studied by difference fluorescence line narrowing

The electron-phonon and vibronic couplings governing the spectral properties have been studied in the Fenna-Matthews-Olson (FMO) bacteriochlorophyll a (BChl a)-protein complex at 4.5 K using a spectrally selective difference fluorescence line-narrowing technique. The complex is a part of the light-harvesting system of green photosynthetic bacteria. Its lowest-energy absorption band, peaking at 826 nm and responsible for the fluorescence, is believed to be due to Q_y transitions of largely isolated molecules. One of the main merits of the used method compared with the more common fluorescence line narrowing is that the zero-phonon lines (ZPL) resonant with the excitation laser can be accurately measured, allowing precise determination of the Huang-Rhys (HR) factors, the main characteristics of the linear electron-phonon and vibronic coupling strengths. Over 60 individual vibrational modes of intra- and intermolecular origin have been resolved in the energy range of 45-1600 cm⁻¹. The small HR factors for these modes, ranging between 0.001 and 0.018, add up to a value of S_vib=0.38±0.07. The effective HR factor for the phonons, S_ph, was found clearly wavelength-dependent, varying from ∼0.7 at short wavelengths to ∼0.3 at the long-wavelength tail of the absorption spectrum. Coupling between the BChl a molecules is likely responsible for this wavelength dependence.     

Probing of the spectral diffusion in doped glasses by fluctuations of dichroism

A method of studying spectral diffusion using temporal fluctuations of the dichroism of an isotropic sample is described. Unlike conventional methods of absorption spectroscopy of single molecules, this method is easier to realize, provides direct information about the distribution of two-level systems over the relaxation rate, and does not require that the absorption lines be Stark modulated in frequency. The latter fact means that this method is applicable to conducting matrices (e.g., for studying current-induced relaxations in polymer materials). In combination with the probing of spectral diffusion using fluorescence intensity fluctuations, this method allows one to determine the degree of correlation between the distributions of impurity centers over the relaxation rate and over the fluorescence quantum yield.     

Doppler spectroscopy of zero-phonon lines

Narrow profiles of purely electronic zero-phonon lines open up attractive possibilities for spectral investigation with the use of the Doppler effect (Doppler spectroscopy). It is established that, for a 10-MHz-wide zero-phonon line to shift to a new position, the velocity of motion of a receiver with respect to a source needs to be only ～10 m/s. It is shown that the narrower the line or the spectral hole (its negative image), the more efficient the Doppler spectroscopy. The possibility of measuring the shape of spectral holes with the use of Doppler spectroscopy is demonstrated, and the potentialities of supersensitive Doppler spectroscopy are considered. The specific feature revealed in the frequency modulation by Doppler spectroscopy is that it occurs without a perturbation of the internal states of the emitter and absorber. The conclusion is drawn that there is a need to study the interference of a photon with itself when the frequency of “one-half of it” changes with the use of Doppler spectroscopy.     

Optical absorption spectra of synthetic CoCO3 single crystal

New results have been obtained on absorption spectra of synthetic CoCO₃ single crystal in the spectral range from the 11,500 to 34,500 cm^-1. Some vibration frequencies of the (CO₃)^2- ions have deduced from the absorption spectrum and zero-phonon lines have been identified. In the regions 22,900 and 24,500 cm^-1, a shift is observed of absorption frequencies with antiferromagnetic order for temperatures increasing from 4.2 to 20°K.     

Low-Temperature Spectral Dynamics of Single TDI Molecules in n-Alkane Matrixes

We report on studies of the influence of the matrix on the spectral dynamics of the zero-phonon-line (ZPL) emission by means of single molecule spectroscopy at low temperature. The host-guest system combinations consist of terrylenediimide (TDI) molecules embedded in four n-alkane matrixes of hexane, heptane, pentadecane, and hexadecane. Excitations into the broad vibronic absorption band and spectrally dispersed detection allows us to monitor fluorescence of single TDI molecules as a function of time. In the case of long-chain n-alkanes (pentadecane and hexadecane), the ZPL line is quite stable, showing spectral jumps of moderate frequency of less than 10 cm(-1) with an average time between the jumps of 10 s. In a clear contrast, the spectral dynamics of TDI molecules embedded within the short-length n-alkane matrixes (heptane and hexane) feature much more frequent spectral jumps that occur on a broader energy scale. The results suggest that matrixes composed of short-chain molecules are more susceptible to translations and/or rotations, which influence the fluorescence of single guest chromophores.     

On the theory of ultranarrow pure-electronic zero-phonon lines

A pure-electronic zero-phonon line, which represents an optical analog of the Mössbauer γ-resonance line, is a cornerstone of three remarkable areas of spectroscopy and optics of impurity solids, namely, ultrahigh-resolution spectroscopy, persistent hole-burning spectroscopy, and spectroscopy of single impurity molecules. In recent experiments made by the photon echo technique, a zero-phonon line width as small as 78 Hz was achieved (with the Q-factor, i.e., the ratio of the transition frequency to the line width being 2×10¹⁴:78≈ 10¹²), which is several tenfold lower than the frequency shift of the vibronic spectrum caused by the momentum of the absorbed or emitted photon of optical range. We estimate the position, intensity, and width of zero-phonon lines under conditions when these characteristics, as well as properties of the phonon wing, are determined not only by Stokes losses but also by the photon recoil effect. It is shown that the latter effect, within the approximation used, does not lead to strong and easily detectable phenomena. They, however, take place, and their study is becoming more topical with improving accuracy and sophistication of the experimental technique (in the photon echo configurations, etc.).     

Phonon-induced exciton dephasing in quantum dot molecules

A new microscopic approach to the optical transitions in quantum dots and quantum dot molecules, which accounts for both diagonal and nondiagonal exciton-phonon interaction, is developed. The cumulant expansion of the linear polarization is generalized to a multilevel system and is applied to calculation of the full time dependence of the polarization and the absorption spectrum. In particular, the broadening of zero-phonon lines is evaluated directly and discussed in terms of real and virtual phonon-assisted transitions. The influence of Coulomb interaction, tunneling, and structural asymmetry on the exciton dephasing in quantum dot molecules is analyzed.